The present invention relates to an aperture piece to be used in conjunction with backscatter apparatus for non-destructively measuring the thickness of a coating on a substrate.
Backscatter instruments are used to measure thickness of coatings on substrates by irradiating the coating with radiation from a radioactive isotope and counting the particles backscattered from the coatings. The backscatter count provides an indication of the coating thickness. One type of such backscatter instrument is shown and described in U.S. Pat. No. 4,155,009.
In using backscatter instruments, the radiation from the radioactive isotope is directed to the workpiece being measured through an aperture in an aperture piece. The function of the aperture piece is to limit the radiation reaching the workpiece to that radiation permitted to travel through the aperture of the aperture piece thereby ensuring that the same amount of radiation reaches the workpiece from measurement to measurement. A description of a workpiece support and mask (aperture piece) assembly for a backscatter measuring instrument is described in U.S. Pat. No. 3,456,115.
The aperture pieces are conventionally designed with a planar sample support surface for interfacing with the workpiece. In backscatter measurements it is important to have the support surface as flat as possible against the workpiece surface.
With aperture pieces now on the market, workpieces having flat surfaces or surfaces with large radii of curvature are easily measured. As the radii of curvature of the workpiece becomes increasingly smaller more difficulties are encountered in maintaining the planar mating relationship between the sample support surface and the workpiece surface. With convex surfaces conventional aperture pieces may be used to make measurements but the measurements are increasingly inaccurate with smaller radii of curvature because of the difficulty in maintaining or closely approximating the desirable planar mating relationship between the sample support surface and the workpiece surface.
In the electronics industry there has risen a need to measure the thickness of coatings on concave surfaces, for example, the inside surfaces of connectors for receiving connector pins. These surfaces typically have inside diameters in the range of 15 mils to 80 mils. It has been a problem in the industry to measure coating thickness on these interior surfaces. One way to measure these surfaces is to break the hollow connector longitudinally and measure one of the pieces on the concave surface. However, it has long been a problem to measure concave surfaces with these small radii of curvature. The problem results from two difficulties--one, the radii of curvature are so small that the planar mating relationship between the sample support surface of the aperture piece and the workpiece surface has been difficult to obtain, and two, it has been difficult to physically position the sample support surface adjacent to the concave surface because of the interference of the aperture piece with the concave workpiece.